Compost Magazine

Composting tips, advice and science.

Lime Increases Compost Speed and Maturity Finds Study

Back in the 19th century, Albert Howard, considered one of the fathers of composting, was a big fan of adding lime to compost. 

In recent decades, though, the practice has been discouraged by many composters who see it as unnecessary. 

After all, lime is alkaline. While compost goes through some major pH swings, it usually ends up with a fairly neutral or slightly alkaline pH.

Now a new study by Chinese researchers has found that adding lime along with a fertilizer (calcium magnesium phosphate*) could have some major benefits for composting.

The researchers tested adding lime varying in strengths from 0.5% to 1.5%, alongside 10% calcium magnesium phosphate, to a compost made from farm and food waste.

The addition led to some pretty interesting results! The researchers found that there was an increase in beneficial bacteria in the compost, which they believe in turn led to faster compost maturity and higher germination rates.

What’s more, the combination of lime and calcium magnesium phosphate meant more nitrogen was retained in the compost – a process the researchers put down to an increase in the pH level. 

This pH level meant that there were decreases in a harmful gas, nitrous oxide, which is one of the ways nitrogen escapes from compost. That meant there were two benefits – a more nutritious compost and reduced greenhouse gases. 

It wasn’t the only reduction in gases either.  

The researchers noted that compost with 0.5% and 1.0% lime showed large reductions in methane and ammonia emissions – two gases which are harmful to the environment. It did this by limiting the growth of bacteria that produce these gases.

Going up to 1.5% lime, though, did cause problems with the amount of hydrogen sulfide – a harmful gas that smells like rotten eggs – starting to increase. 

On balance, though, it does sound like Albert Howard may have been on to something with the addition of lime, so should we add it?

Possibly not yet, at least for home composters. The benefits this study found came about when the lime was combined with another additive – and at 10%, you’d need quite a lot of it if you have a large compost heap. 

Not all of us have access to calcium magnesium phosphate in quantity, or need to spend the extra money needed to add additives. 

It’s always worth being careful before changing practices based on a single study. 

Still, it’s a reminder to keep an open mind when it comes to composting practices – and be ready to learn from research! In the meantime, if you want to improve your composting, check out these 17 ways to improve compost speed

* Calcium magnesium phosphate is a slow-release fertilizer that contains calcium, magnesium, and phosphorus.


Zhang, L., Shi, T., Xu, Z., Bao, Z., Li, J., Li, G., & Yuan, J. (2023). Effect of lime and calcium magnesium phosphate on gaseous emissions, maturity, and bacterial dynamics during food waste composting. Environmental Technology & Innovation, 103306.

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